1. Field of the Invention
This invention is concerned with analysis of organic additives and other components of plating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Electroplating baths typically contain organic additives whose concentrations must be closely controlled in the low parts per million range in order to attain the desired deposit properties and morphology. One of the key functions of such additives is to level or brighten the deposit by suppressing the electrodeposition rate at peaks in the substrate surface. Leveling/brightening of the deposit results from faster metal deposition within recessed areas where the additive, which is present at low concentration, is less effectively replenished by diffusion/bath agitation as it is consumed in the electrodeposition process. The most sensitive methods available for detecting leveling and brightening additives in plating baths involve electrochemical measurement of the metal electrodeposition rate under controlled hydrodynamic conditions for which the additive concentration in the vicinity of the electrode surface is well-defined.
Cyclic voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J. Electrochem. Soc. 125, 194 (1978)] is the most widely used bath additive control method and involves cycling the potential of an inert electrode (e.g., Pt) in the plating bath between fixed potential limits so that metal is alternately plated on and stripped from the electrode surface. Such voltage cycling is designed to establish a steady state for the electrode surface so that reproducible results are obtained. Cyclic pulse voltammetric stripping (CPVS), also called cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in voltage during the analysis to condition the electrode so as to improve the measurement precision [D. Tench and J. White, J. Electrochem. Soc. 132, 831 (1985)]. A rotating disk electrode configuration is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions.
Accumulation of organic films or other contaminants on the electrode surface can be avoided by periodically voltage cycling the electrode in the plating solution without organic additives and, if necessary, polishing the electrode using a fine abrasive. The metal deposition rate can be determined from the current or charge passed during metal electrodeposition but it is usually advantageous to measure the charge associated with anodic stripping of the metal from the electrode. The CVS method was first applied to control copper pyrophosphate baths (U.S. Pat. No. 4,132,605 to Tench and Ogden) but has since been adapted for control of a variety of other plating systems, including the acid copper sulfate baths that are widely used by the electronics industry [e.g., R. Haak, C. Ogden and D. Tench, Plating Surf. Fin. 68(4), 52 (1981) and Plating Surf. Fin. 69(3), 62 (1982)].
Acid copper sulfate electroplating baths require a minimum of two types of organic additives to provide deposits with satisfactory properties and good leveling characteristics. The suppressor additive is typically a polymeric organic species, e.g., high molecular weight polyethylene or polypropylene glycol, which adsorbs strongly on the copper cathode surface to form a film that sharply increases the overvoltage for copper deposition. This prevents uncontrolled copper plating that would result in powdery or nodular deposits. An anti-suppressor additive is required to counter the suppressive effect of the suppressor and provide the mass-transport-limited rate differential needed for leveling. Plating bath vendors typically provide additive solutions that may contain additives of more than one type, as well as other organic and inorganic addition agents. The suppressor additive may be comprised of more than one chemical species and generally involves a range of molecular weights.
Both the suppressor and the anti-suppressor additive concentrations in acid copper sulfate baths can be determined by CVS analysis methods based on the effects that these additives exert on the copper electrodeposition rate. For the suppressor analysis, the CVS rate parameter, usually the copper stripping peak area at a given electrode rotation rate (Ar), is first measured in a supporting electrolyte having approximately the same composition as the plating bath to be analyzed but without organic addition agents. Additions of known volume ratios of the plating bath to the supporting electrolyte (or to a background electrolyte having known concentrations of other additives) produce decreases in the CVS rate parameter that reflect the concentration of the suppressor additive. This xe2x80x9cstandard additionxe2x80x9d suppressor analysis is not significantly affected by the presence of the anti-suppressor, which exerts a relatively weak effect on the copper deposition rate at the plating bath dilution levels involved. For the anti-suppressor analysis, a sufficient amount of the suppressor additive, which may be comprised of a plurality of components or species, is added to the supporting electrolyte to produce a background electrolyte exhibiting substantially the maximum suppression of the copper deposition rate (minimum CVS rate parameter). Additions of known volume ratios of the plating bath to this fully-suppressed background electrolyte produce increases in the CVS rate parameter that can be related to the concentration of the anti-suppressor additive. The exact procedures for CVS analysis of acid copper sulfate baths can vary.
Analysis for the suppressor additive (also called the xe2x80x9cpolymerxe2x80x9d, xe2x80x9ccarrierxe2x80x9d, or xe2x80x9cwetterxe2x80x9d, depending on the bath supplier) typically involves generation of a calibration curve by measuring the CVS rate parameter Ar in a supporting or background electrolyte (without organic additives or with known concentrations of interfering additives), termed Ar(0), and after each standard addition of the suppressor additive. For the calibration curve, Ar may be plotted against the suppressor concentration directly, or normalized as Ar/Ar(0) to minimize measurement errors associated with changes in the electrode surface, background bath composition, and temperature. For the suppressor analysis itself, Ar is first measured in the supporting electrolyte and then after each standard addition of a known volume ratio of the plating bath sample to be analyzed. The suppressor concentration may be determined from the Ar or Ar/Ar(0) value for the measurement solution (supporting electrolyte plus a known volume of plating bath sample) by interpolation with respect to the appropriate calibration curve (xe2x80x9cresponse curve analysisxe2x80x9d). Alternatively, the suppressor concentration may be determined by the xe2x80x9cdilution titrationxe2x80x9d method from the volume ratio of plating bath sample (added to the supporting electrolyte) required to decrease Ar or Ar/Ar(0) to a given value, which may be a specific numerical value or a minimum value (substantially maximum suppression) [W. O. Freitag, C. Ogden, D. Tench and J. White, Plating Surf. Fin. 70(10), 55 (1983)]. Note that the effect of the anti-suppressor on the suppressor analysis is typically small but can be taken into account by including in the background electrolyte the amount of anti-suppressor measured or estimated to be present in the plating bath being analyzed.
The concentration of the anti-suppressor additive (also called the xe2x80x9cbrightenerxe2x80x9d, xe2x80x9cacceleratorxe2x80x9d or simply the xe2x80x9cadditivexe2x80x9d, depending on the bath supplier) is typically determined by the linear approximation technique (LAT) or modified linear approximation technique (MLAT) described by R. Gluzman [Proc. 70th Am. Electroplaters Soc. Tech. Conf., Sur/Fin, Indianapolis, Ind. (June 1983)]. The CVS rate parameter, Ar, is first measured in background electrolyte containing no anti-suppressor but with a sufficient amount of suppressor species added to substantially saturate suppression of the copper deposition rate. A known volume ratio of the plating bath sample to be analyzed is then added to this fully-suppressed background electrolyte and Ar is again measured. The Ar measurement is then repeated in this mixed solution after each addition (typically two) of known amounts of the anti-suppressor additive only. The concentration of the anti-suppressor in the plating bath sample is calculated assuming that Ar varies linearly with anti-suppressor concentration, which is verified if the change in Ar produced by standard additions of the same amount of anti-suppressor are equivalent.
Acid copper sulfate baths have functioned well for plating the relatively large surface pads, through-holes and vias found on printed wiring boards (PWB""s) and are currently being adapted for plating fine trenches and vias in dielectric material on semiconductor chips. The electronics industry is transitioning from aluminum to copper as the basic metallization for semiconductor integrated circuits (IC""s) in order to increase device switching speed and enhance electromigration resistance. The leading technology for fabricating copper IC chips is the xe2x80x9cDamascenexe2x80x9d process (see, e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p. 32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.), which depends on copper electroplating to provide complete filling of the fine features involved. The organic additives in the bath must be closely controlled since they provide the copper deposition rate differential required for bottom-up filling.
As the feature size for the Damascene process has shrunk below 0.2 xcexcm, it has become necessary to utilize a third organic additive in the acid copper bath in order to avoid overplating the trenches and vias. Note that excess copper on Damascene plated wafers is typically removed by chemical mechanical polishing (CMP) but the copper layer must be uniform for the CMP process to be effective. The third additive is called the xe2x80x9clevelerxe2x80x9d (or xe2x80x9cboosterxe2x80x9d, depending on the bath supplier) and is typically an organic compound containing nitrogen or oxygen that also tends to decrease the copper plating rate. In order to attain good bottom up filling and avoid overplating of ultra-fine chip features, the concentrations of all three additives must be accurately analyzed and controlled.
The concentrations of the suppressor and anti-suppressor in acid copper plating baths can be analyzed with good precision in the presence of the leveler additive by the standard CVS methods. At the additive concentrations typically employed, the effect of the suppressor in reducing the copper deposition rate is usually much stronger than that of the leveler so that the concentration of the suppressor can be determined by the usual CVS response curve or dilution titration analysis. Interference from the leveler can be minimized by utilizing a background electrolyte for the suppressor analysis that contains approximately the same leveler concentration as in the plating bath being analyzed, estimated from the bath makeup composition and previous analyses. Likewise, the anti-suppressor concentration can be determined by the LAT or MLAT analysis procedure and the approximate bath concentration of leveler can be added to the fully-suppressed background electrolyte to minimize leveler interference. With some modifications, for example, to account for relatively high leveler activity or to reduce anti-suppressor interference on the suppressor analysis, these CVS procedures provide reliable measures of the suppressor and anti-suppressor additives used in currently-available acid copper electroplating baths. In addition, a method for measuring the leveler concentration in the presence of interference from both the suppressor and anti-suppressor was described in U.S. patent application Ser. No. 09/968,202, filed Oct. 1, 2001, to Chalyt et al., which is assigned to the same assignee as the present application.
For CVS and other voltammetric bath analysis methods based on measurements of the metal electrodeposition rate, the electrode potential must be precisely controlled. This is normally accomplished by use of a reference electrode in conjunction with an electronic potentiostat. However, the potential of commercially available reference electrodes tends to drift with time, especially as plating bath chemicals diffuse into the reference electrode solution. Some commercial reference electrodes employ gelled electrolytes to inhibit diffusion of contaminants but such electrodes exhibit significant potential drift both under storage conditions and in contact with plating baths. Reference electrode voltage drift can introduce large errors in measured metal deposition rate parameters. For example, a small change in the cathodic potential limit, at which the metal deposition current is highest, has a large effect on the overall amount of metal deposited and consequently on the metal stripping peak areas typically used for CVS and CPVS additive analyses. Likewise, the cathodic current associated with metal deposition typically increases sharply with increased cathodic potential in the region of interest for voltammetric bath analysis so that a small error in measured electrode potential has a large effect on the measured current.
The normal procedure for handling reference electrode drift is to replace the test reference electrode or make corrections to the measured potential based on periodic calibration of the test reference electrode against that of a standard reference electrode. Such calibration involves placing the two electrodes in contact with an electrolyte and measuring the potential difference using a high-impedance voltmeter. Ideally, the two reference electrodes are of the same type and the electrolyte used for the calibration is the same as that in the electrodes so that junction potentials and contamination of the solution in the standard reference are avoided. Alternatively, contamination by plating bath species can be minimized by bringing the standard reference electrode into contact with the plating bath or the supporting electrolyte for only brief periods of time.
Use of a standard reference electrode for calibration of the test electrode generally involves periodic removal of the test electrode from the analysis equipment or insertion of the standard reference electrode in a plating solution in the analysis equipment, which are typically manual operations that are time-consuming and costly. In addition, periodic calibration against another standard reference electrode is needed to ensure that the potential of the standard reference electrode used to calibrate the test electrode remains constant. Interruption of bath analysis during the time required for reference electrode calibration or changeout of reference electrodes can also present a problem, particularly for automated on-line analysis equipment designed to provide very close control of critical electroplating process parameters. Access to the reference electrode in such automated equipment is often not very good, rendering calibration more difficult and time consuming. The only approach currently available for addressing these problems is to use complicated equipment that automatically changes the solution in the reference electrode.
There is an important need for a method of calibrating reference electrodes used for plating bath analyses that does not require removal of the electrode from the plating equipment and can be performed automatically and quickly without complicated equipment. In addition to saving labor, time and expense, such a method would make frequent reference electrode calibration practical so that measurement errors could be minimized.
The present invention is a method of calibrating the reference electrode used for voltammetric analysis of a plating bath. In this method, an inert working electrode and the reference electrode are brought into contact with the supporting electrolyte of the plating bath (or the plating bath itself or a background electrolyte) and the potential of the working electrode is changed as a function of time relative to the potential of the reference electrode such that metal is plated onto and then anodically stripped from the working electrode surface. The current response to the potential of the working electrode is monitored and the potential corresponding to a selected stage in the current response is used to calibrate the reference electrode potential. The calibration is preferably performed using the supporting electrolyte rather than the plating bath or a background electrolyte to avoid interference from plating bath additives.
Various stages in the current response are apparent in plots of current versus working electrode potential. The key features of such voltammograms are the cathodic current associated with metal plating and an anodic peak associated with substantially complete stripping of the metal from the working electrode surface. The stage of the current response used for the calibration is selected such that the corresponding working electrode potential is substantially independent of normal variations in the supporting electrolyte composition and temperature. A preferred stage in the current response for reference electrode calibration is the zero-current crossover from metal plating to metal stripping but other current stages may also be used.
In a preferred embodiment, the working electrode is an inert metal (platinum, for example) in the well-known rotating disk configuration and is rotated (typically at a constant rate) to control solution mass transport so as to provide more reproducible results. Also, it is usually advantageous to use a counter electrode and an electronic potentiostat to control the potential of the working electrode relative to the reference electrode. This approach avoids passing appreciable current through the reference electrode, which could polarize the reference electrode and change its potential. In some cases, especially when the currents involved are relatively small, the reference electrode may also serve as the counter electrode so that a separate counter electrode is not needed. In one embodiment, the potential of the working electrode relative to the reference electrode is cycled at a constant rate between fixed negative and positive potential limits, as in the CVS bath analysis method, but other voltage waveforms may be used. For example, the working electrode potential may be scanned in some potential regions and stepped in others, as in the CPVS bath analysis method. It is usually advantageous to employ a plurality of potential cycles between fixed limits to provide a steady-state electrode surface, which typically yields more reproducible results. Steady-state is indicated by substantially equivalent voltammograms on successive cycles.
The reference electrode calibration of the present invention is readily performed during the normal course of CVS or CPVS plating bath analysis. For example, the test reference electrode may be calibrated relative to the zero current point between plating and stripping using the same voltammetric data generated to determine the stripping peak area Ar(0) in the supporting electrolyte for the CVS or CPVS analysis. Since the plating bath analysis is typically performed under computer control, no additional equipment is needed to automatically perform the calibration of the present invention. Such automatic calibration of the reference electrode without removal from the plating equipment, which is enabled by the present invention, saves labor, time and expense, and minimizes errors in the plating bath analysis.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.